1. Technical Field
The present disclosure relates generally to compositions of ovonic threshold switches.
2. Description of the Related Art
A phase change memory device includes an array of memory cells, where each memory cell includes a memory element and a selection element. The memory element is a phase change material, i.e., a chalcogenide alloy that may be electrically switched between a generally amorphous and a generally crystalline state or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. The state of the phase change material is also non-volatile in that, when set in a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until changed by another programming event. The resistance value that is retained represents a phase or physical state of the material (e.g., crystalline or amorphous). This physical state of the phase change material is unaffected by removing electrical power.
The selection element may also be made of a chalcogenide alloy, however, unlike the memory element, the selection element does not change phase. More particularly, the selection element, which may be referred to throughout as an ovonic threshold switch, an OTS, or a switching device, does not exhibit a phase change from amorphous to crystalline. An ovonic threshold switch toggles between an “on” and “off” state depending on the amount of voltage potential applied across the phase change memory cell. The state of the ovonic threshold switch changes when a current through the ovonic threshold switch exceeds a threshold current or voltage. Once the threshold current or voltage is reached, the on state is triggered and the ovonic threshold switch is in a substantially conductive state. If the current or voltage potential drops below a threshold value, the ovonic threshold switch will return to the off state.
The behavior (including switching, memory, and accumulation) of chemical compositions of chalcogenide materials have been described, for example, in the following U.S. Pat. Nos. 6,671,710; 6,714,954; 6,087,674; 5,166,758; 5,296,716; 5,536,947; 5,596,522; 5,825,046; 5,687,112; 5,912,839; and 3,530,441, the disclosures of which are hereby incorporated by reference. The references also describe the structural transformations from the crystalline state to the amorphous state (and vice versa), including a series of partially crystalline states in which the relative proportions of crystalline and amorphous regions may vary during the operation of electrical and optical chalcogenide materials.
The phase change material is typically formed with one or more chalcogenide elements (i.e., Group VI elements from the periodic table). Examples of chalcogenide materials are described in U.S. Pat. Nos. 5,166,758, 5,296,716, 5,414,271, 5,359,205, 5,341,328, 5,536,947, 5,534,712, 5,687,112, and 5,825,046 the disclosures of which are all incorporated by reference herein.